Imaging systems such as printers, fax machines, and copiers are virtually omnipresent, and can be found in homes and offices worldwide. The development of such systems has facilitated improvements in communication that have in turn fostered a sea of change in the way people live and work. Telecommuting, paperless offices, and intra-office networks represent but a few examples of the advancements that have been made possible by modern imaging systems.
Since these systems have become crucial to everyday existence, their reliability and smooth operation is paramount. It is therefore vitally important to design imaging systems so that downtime and work interruptions are minimized. This can be a daunting challenge, given the relative complexity of systems in which sheet material must be infed, moved through the imaging process, and outfed in a matter of seconds.
It has been found that the difference in weight between smaller and larger sheet sizes, differences in weight between thicker and thinner sheets, and different sheet surface textures can present problems in sheet feeding throughout the imaging system. For each combination of these factors, successful transportation of sheet material depends upon applying the proper amount of applied force on the sheet with a feed mechanism such as a roller. The combination of forces is shown in FIG. 1. When torque T is applied to the roller R, the combination of torque and normal force N produces a transport force P, which causes the sheet material to move. Accurate application of the applied force A transmitted through the roller R allows the system designer to produce the desired normal and transport forces appropriate for a particular sheet material.
The consequences of incorrect forces can be problematic. Using the infeed mechanism as an example, if the applied force is too low, sheets can have "no-pick" problems, where the transport force is insufficient to remove the sheets from the stack. At the other end of the spectrum, if the force is too great, the result may be "multi-feed" problems, wherein the transport force introduces several sheets into the feed mechanism simultaneously. Excess force can also cause deformation of one or more of the underlying sheets.
Among the known approaches to address these difficulties are systems which rely upon a method of sensing the height of a stack of sheet media, then placing the transport mechanism based on the sensed height of the media stack. Such systems are relatively complex, and do not directly control the normal force applied to the media stack when the pick mechanism operates. Further, known devices employ multiple sensing devices, one to sense stack height and another to sense transport mechanism position.
It can thus be seen that the need exists for a reliable and predictable way to transmit applied force to a feed roller associated with imaging devices such as printers, copiers, and fax machines.